Matter and antimatter. Why are we here?

The matter making the solar system, the Earth, all living beings and ourselves, is made up almost entirely of atoms which, in turn, are based on three elementary particles: protons, neutrons and electrons. For each of these particles, as well as for many others, not usually part of atoms, there is an antiparticle. Therefore, there could be antimatter antiatoms, made of antiprotons, antineutrons, and antielectrons (positrons).

An interesting property of matter and antimatter is that they cannot be together. As soon as they come into contact, they completely disintegrate, transforming into energy. Everything suggests that our galaxy (the Milky Way) is made up almost exclusively of matter. There is also some antimatter, in the form of antiparticle clouds, outside the galaxy, close to it and attracted by its gravity, but in such a small quantity, compared to the mass of the galaxy, that for practical purposes it can be ignored. It has also been said that there could be some (but very few) anti-stars.

The Hubble-Lemaître Law

Georges Lemaître

Let us look at a little history.

In various places in the sky, but especially in the constellation of Cepheus, where the first case was discovered, there are some stars whose light intensity varies regularly, and therefore are called variable Cepheids. In 1908, the American astronomer Henrietta Swan Leavitt discovered that the period of these stars is linked with their real luminosity: the greater the luminosity, the longer the period. Therefore, by measuring their period, their real luminosity can be deduced.

In 1913, the American astronomer Vesto Melvin Slipher obtained the spectrum of what was then called the Andromeda nebula (the giant galaxy closest to ours) and discovered a blue shift that indicated (according to the Doppler effect) that the nebula moves towards us with a speed of about 300 kilometers per second, much higher than expected. Slipher then studied the light of other spiral nebulae and made the unexpected discovery that most of them, unlike Andromeda, show redshifts, that is, they move away from the solar system with great speed. In fact, he measured speeds above 1000 kilometers per second.

In 1919, the American astronomer Edwin Powell Hubble used the Mount Wilson telescope to photograph several spiral nebulae, including Andromeda, and showed that, actually, they were not nebulae, as had been believed, but huge clusters of stars. From then on they were no longer called nebulae, but galaxies, in honor of our Milky Way, which also belongs to the class of spiral galaxies. Galactos in Greek means milk.